Initiator assembly



Dec. 13, 1960 c. F. HORNE 2,963,971

INITIATOR ASSEMBLY Filed Oct. 2. 1957 H 25 24 I0 I l4 I6 22 I9 3 CHARLESE HORNE z v INVENTOR.

FIG.9 BY

AGENT.

United States Patent INITIATOR ASSEMBLY Charles F. Home, Kingston, N.Y.,assignor to Hercules Powder Company, Wilmington, Del., a corporation ofDelaware Filed Oct. 2, 1957, Ser. No. 687,718

19 Claims. 01. 102-28) This invention relates to improved electricinitiators. In still another aspect, this invention relates to electricdelay blasting caps that can be fired uniformly by current directly fromhigh voltage power lines. In another aspect, this invention relates toan electric delay blasting cap assembly containing a fuse element in atleast one leg wire for preventing damage to the cap together withconcomitant failure, when high voltage currents are utilized. In anotheraspect this invention relates to fuse structure in at least one leg wireof an electric delay blasting cap, characterized by a tensile strengthat least as high as that of the leg wire with which it is associated. Instill another aspect this invention relates to a method for utilizinghigh voltage currents in firing electric delay blasting caps.

As is well known, an electric delay blasting cap comprises a shell, adetonating charge, a delay fuse in close proximity to the detonatingcharge, or to a primer which in turn is disposed in close proximity tothe detonating charge, leg wires within the shell and connected thereinat their terminal ends by a resistance or bridge wire at a point abovethe delay fuse, and at their other ends with lead wires from the powersource, and an ignition composition around or in close proximity to thebridge wire which is ignitible by heat from the wire upon passage ofelectric current therethrough so that by its ignition it sets offburning of the fuse followed by detonation of the base charge, or theprimer and then the base charge as the case may be.

Ordinarily, electric blasting caps are fired by a battery charge, or ablasting machine which is essentially a handdriven dynamo and whichdelivers a short duration of current, e.g., for 20-25 ms. and exhibits avoltage generally in the order of about 100-400. Very frequently,however, blasting machines, batteries, or other ordinary current sourcesare not available at the scene of operations and resort is had to theavailable power line sources, generally 220 or 440 volts, which supplyexcessive amperage and power for firing electric blasting caps.

When so utilizing a high voltage power line there is a great tendencyfor the formation of an arc within the casing of the blasting cap whichby development of heat, causes high pressure within the cap andconsequently bursting of the cap shell before the slow burning fuse hasburned down to the detonating charge or sometimes before it has beenignited. Such bursting of the cap shell has the effect of destroying thecap shell, moving the burning fuse away from the primer, or detonatingcharge, and for allowing the entrance of water into the delay, therebycausing failure to detonate, or misfires of the dynamite charge in theborehole.

Such arcing may occur along the path of the volatilized bridge wireafter the bridge wire has been melted by the heat from the currentpassing through it. Areing may also occur within the cap shell if thetwo lead wires are in too close proximity to one another or to the capshell. In either case, the result is the bursting of the cap shell andscattering of the cap elements by "ice the internal pressure, so thatthe fuse has either failed to ignite or it is extinguished after it isignited.

This invention is concerned with an electric delay blasting cap assemblycontaining a fuse device for permitting use of high voltage firingcurrents without the occurrence of arcing and bursting of the cap shelland concomitant failure of the delay fuse to set off the detonating orbase charge.

An object of this invention is to provide improved electric initiators.Another object is to provide a delay blasting cap especially adapted tofiring by high voltage currents. Another object is to provide a novelfuse assembly in an electric delay blasting cap for the elimina tion ofarcing of the firing current in the blasting cap and concomitantbursting and failure of the cap. Another object is to provide a methodfor utilization of high, voltage currents in the firing of electricdelay blasting caps. Other aspects and objects will be apparent in lightof the accompanying disclosure and the appended claims.

In accordance with the invention there is provided an improvement in anelectric delay initiator assembly containing leg wires within the capshell, a bridge wire in the cap shell connecting the said leg wires, andassoci ated ignition, delay and base charge compositions, the saidimprovement permitting high voltage current firing without loss of thecap from bursting, and comprising a fusible electrical conductor in atleast one of the leg wires, within the cap shell; the said conductor andthe bridge wire each being characterized by a fusion point such that itwill fuse within a period of milliseconds when the initiator is fired ata voltage sufiiciently high to deliver a current of at least 10 amperesto the bridge wire; said bridge wire being fusible during said firingprior to fusion of the said conductor. Further in accordance with theinvention an improvement is provided in a method for the utilization ofhigh voltage currents in firing electric delay blasting caps whichimprove ment comprises, maintaining an electrical conductor section inat least one of the leg wires, within the cap shell, having a fusionpoint such that it will fuse within a period of 120 milliseconds whenfiring the initiator at a voltage sufiiciently high to deliver a currentof at least 10 amperes to the bridge wire, and utilizing a bridge wirehaving a fusion point such that it willalso fuse within 120 millisecondswhen said initiator is fired as described, but prior to fusion of saidconductor, whereby flow of current through said leg wire is terminatedprior to the occurrence of sufficient arcing to cause bursting of thecap shell and concomitant failure of detonation of the base charge.

The invention is illustrated with reference to the attached drawings ofwhich Figure 1 is a front elevational view of an electric delayinitiator assembly of the invention, and Figures 29 are illustrative ofvarious embodiments of conductor or fuse elements which can be insertedin one or both leg wires of the assembly to provide adequate tensilestrength so that the lead wire is not pulled apart during handling ofthe cap due to the limited strength of the fuse element itself.

With reference to Figure 1, shell 9 of electric delay blasting cap 10,generally a metal, can be of any suitable material. Leg wires 11 inshell 9 connect with lead wires from a power source (not shown) and eachcontains a conductor section 12, preferably within ignition plug 8. Legwires 11 are connected in shell 9 at the terminal ends 14 below plug 8,with a resistance or bridge wire 16. A suitable primer 15 such asdiazodinitrophenol is superposed on a base charge 17 such aspentaerythritol tetranitrate (PETN) in closed end 18 of shell 9. Delayfuse 19 comprises any suitable assembly, such as a lead tube 21containing core 22 of a suitable delay powder, e.g., BaO /Se, superposedon primer 15.

An ignition or flash composition 20 is disposed in operativecommunication with bridge wire 16 so as to be ignited by heat from wire16 resulting from passage of electric current by way of leg wires 11.Leg wires 11 extend upwardly from bridge wire 16 in shell 9 throughignition plug 8 through a sulfur plug 24, a suitable waterproofing plug25 and a sulfur topping plug 26 and are joined in any suitable mannerwith a power source outside shell 9 including a source of high voltagecurrent.

Conductors, or fuses, 12 which each comprises a link in leg wire 11within shell 9 are characterized by a fusion point such that they fusewithin a period of about 120 milliseconds (ms.) when the cap is fired ata voltage sufficiently high to deliver a firing current of at leastamperes to the bridge wire. Bridge wire 16 is also characterized by afusion point such that it will also fuse within 120 milliseconds underthe said firing conditions except that, under those conditions, it willfuse prior to the fusion of conductors 12. Thus, if bridge wire 16 andfuse elements 12 are of the same alloy, the crosssectional diameter ofbridge wire 16 will be somewhat smaller than that of each fuse 12. Inany event, it is a matter of selecting bridge wire 16 and fuse element12 in respect of their relative fusion points, specific resistance, andthe like. Fuse elements 12 and bridge wire 16 are selected so as to fusepreferably within 35-100 milliseconds after the high voltage firingcurrent is applied.

Bridge wire 16 is advantageously a plain or gold-plated Nichrome wire(nickel/chromium/iron, 60/16/24), a diameter of about 0.00175 inch beingrepresentative of the preferred wire thickness. Further exemplary is aplatinum/rhodium/ruthenium wire 79/15/6 having a diameter of about 0.001inch.

The cross section of the fuse wire 12 is, of course, selected consonantwith the size and fusion point of the bridge wire 16. For example thediameter of a Nichrome fuse wire will be somewhat larger than the0.00175 value of a Nichrome bridge wire, such as about .0056 inch indiameter.

In the preferred embodiment of the structure of this invention, thebridge wire and the conductor or fuse wire are of the same alloy, thediameter of the conductor wire being generally from about 0.001 to 0.008inch greater than that of the bridge wire.

It is an important feature of the invention that the conductor wire andthe bridge wire both fuse within a period of about 120 milliseconds.When longer periods are employed, that is to say, when the current flowsfor a period longer than 120 milliseconds, sufficient temperature hasdeveloped within the blasting cap as to cause development of sufficientheat and pressure to bring about bursting of the shell and failure ofthe detonation.

The following alloys are representative of those particularlyadvantageously applied as either or both fuse wire 12 and bridge wire16: Nichrome, karma, advance, platinum alloy, or any wire normallyutilized for bridge wires in electric blasting caps. The fuse wire neednot be of the same composition as the bridge wire but may be zinc,copper, aluminum or any wire commonly known to the art.

Ignition, delay, primer, and detonator compositions well known in theart for use in delay electric blasting caps can be employed in thepractice of this invention. Thus, exemplary of these compositions areignition compositions such as a loose charge of lead/selenium(72.4/27.6), dipped bridge compositions (matchhead type) such asparaphenylenediamine dipicrate/potassium chlorate/Snow Floss (trade namehighly purified earthy formation of siliceous skeletons of diatoms)24/56/20, lead mononitroresorcinate/potassium chlorate, 80/20, and thelike; delay compositions such as barium peroxideselenium 84/16, bariumperoxide-tellurium, potassium permanganate-selenium, red lead-sulfur,and the like; primer compositions such as diazodinitrophenol, mercuryfulminate, mercury fulminate/potassium chlorate,diazodinitrophenol/potassium chlorate, and the like; and detonator orbase charge compositions such as pentaerythritol tetranitrate,cyclotrimethylene trinitramine (cyclonite), tetryl lead azide, and thelike.

The conductor or fuse wires 12 are preferably disposed within ignitionplug 8 in order that maximum support can be given by the plug assemblyto the leg wires 11 in lieu of the loss in tensile strength that occurswhen the lower tensile strength conductor wire 12 is inserted as asection in the wire 11. However, if desired, conductors 12 can bepositioned outside the ignition plug, but still within the shell 9 suchas within sections 24, 25 or 26, which can be respectively a sulfurplug, a waterproofing plug, and a sulfur topping.

With reference to Figure 2, is shown an ignition plug 8 through whichleg wires 11 extend, and outside of which one of the lead wires 11' isdirectly connected with a leg wire 11 and the other is connected with aleg wire 11 through the fuse wire 12 which is preferably a Nichrome wireabout .0056 inch diameter and A: inch long, having a specific resistanceof about 650 ohms/avg. mil foot. Bridge wire 16 is matched" to wire 12in that upon the passage of high voltage current (delivering at least 10amperes to the bridge wire) through the leg wires and the bridge wire,they are both fused within 120 milliseconds, the bridge wire 16 beingadapted to fuse prior to fusion of wire 12. Figure 2a is similar toFigure 2 except that it is illustrative of a fuse conductor 12 in bothleg wires 11, all disposed within the shell 9 of Figure 1.

With reference to Figure 3, ignition plug contains leg wires 11, asection of one of which is cut away to provide a fuse section 41. Figure3a is similar to Figure 3 except that it illustrates fuse section 41 inboth leg wires 11.

In Figures 4, one leg wire 11 is completely severed and then rejoined bya fuse wire 12. Figure 4a is similar to Figure 4 except that itillustrates a severance of both leg wires 11 and rejoining of each by afuse wire 12.

In the practice of the invention it is important that the fuse conductorexhibit sufficient tensile strength as to support the weight of the capand associated explosive charge during the blasting operation.Frequently, the leg wires support the entire weight of the totalexplosive charge and in such instance the fuse conductor must exhibit atensile strength sufiicient for such support, say in the order of about15 to 45 lb. dead weight, for each wire. I have found that although asoldered-in fuse wire or pair of wires can be utilized as abovedescribed, the tensile strength requirement often limits the size of thefuse conductor that can be used so that some alloys which exhibit highfusion points and high specific resistance values cannot be used becausein such instance the large diameter for the requisite tensile strengthmay preclude heating to the fusion point.

Therefore, I have provided fuse conductors in the form of assembliessupported on a dielectric material by which the necessary tensilestrength is provided by an insulating or dielectric composition whichsupports the fuse conductors free from any specific tensile strengthrequirement. Various embodiments of such type assemblies are illustratedwith reference to Figures 5-8a of the drawings.

With reference to Figure 5, leg wire 11 is disposed in ignition plug 8cand is soldered to one end 51 of copper sheet 50, which sheet is securedto dielectric element 52 to form a laminate. An uninsulated end of theremaining portion of leg wire 11 is soldered to opposite end 51' ofsheet 50. Section 51" of the metal plate 50, intermediate ends 51 and51' is constricted, the constricted portion serving as a conductor fuseof this invention. The copper plate portion is fastened to thedielectric material 52 in any suitable manner such as by adhesion.Figure 5a is the same as Figure 5 except that it shows such a fuse wireassembly associated with both leg wires in the blasting cap. Thus, inFigure 5a, a single layer of dielectric material can serve to supporttwo copper sheets insulated from each other for the purpose ofsupporting leg wires thereto to provide a fuse conductor of thisinvention.

Figure 6 shows a dielectric type assembly in conjunction with anignition plug 8d in which two individual copper strips 53 are afiixed inparallel on the surface of a dielectric member 52 and each of the pairof leg wires 11 is atfixed to one of the individual strips to provide asupported fuse assembly. Thus, the strips 53, each supported ondielectric material 52, function as the fuse conductor in each of theleg wires 11.

Figure 7 illustrates a now preferred assembly which comprises plug 8containing separate leg wire portions 11 soldered to separate coppersheets 54 and 56, each supported on dielectric material 52 and insulatedfrom each other. Copper portions 54 and 56 are connected by a suitablefuse wire conductor 57, which, as shown, is individually supported bydielectric material 52 and can be made of any desirable dimensionindependently ofany ferred paint comprising a suspension of from about30-50 M 6 EXAMPLE I p Firing data were obtained employing a short lengthNichrome wire inch) of 0.0025 inch diameter soldered directly into oneleg wire as a fuse element of the invention. The entire fuse wireelement was sealed within the cap shell by a filling of asphalt,polyester resin (Stypol), or sulfur. The firing was carried out at 220volts with a line resistance of 15 ohms.

Table 1 Test No. N o. of Trials Result of Firing 10 10 Normal-All Shot.10 10 Normal-All Shot. 6 6 Norma1All Shot.

The delay caps fired in each one of these tests employed a looseignition composition around the bridge wire, leadselenium, and a bariumperoxide/selenium delay fuse.

The primer was diazodinitrophenol and the base charge waspentaerythritol tetranitrate (PETN).

EXAMPLE H Several tests, the same as those described with reference toExample I were made, the cap assembly being the same except that aNichrome wire fuse element, A inch in length was soldered in each leadwire, within the cap shell, as illustrated with reference to Figure 2a.One test was made in the absence of any fuse wire as a control. Datasummarizing the tests are set forth in .Table 2.

Table 2 FIRING VOLTAGE, 220 VOLTS Line Re- Current No. in No. of TestN0. Fuse Wire sistgnce, Amperes 1 Parallel Trials Results of Firing 0.21, 100 1 30 Normal, 9H, 200-6 Failed. 2. 0 110 1 30 Normal. 29H-AllShot. 5.0 32.7 1 30 29H 10.-All Shot. 10.0 18.8 1 30 20 ormal, 3H, 7P-8Failed. 4 (Control 0.2 1, 100 5 6 1 Normal, 29H-All Shot.

Test) 0. 2 1, 100 10 5 1 Normal, 42H, 7CAll Shot.

0.2 1, 100 20 3 1 Normal, 56H, 20, 1P-6 Failed. 23. 5 9. 4 1 5 5Normal-All Shot. 25.0 8. 8 1 25 25 Normal-All Shot. 15.0 14.6 1 27 4Normal, 7H, 14F, 20-9 Failed. 2 9. 9 11. 1 1 13-1 Failed.

0. 2 1, 100 1 40 Normal-All Shot. 2.0 110 1 40 D0. 5.0 32. 7 1 40 D0. 5N ichrome 10. 0 18. 8 1 40 Do. 0.2 1,100 5 8 D0. 0.2 1,100 10 4 D0. 0.21,100 20 2 D0. 2% 22. i a se inara orma o Nwhmme 10.0 is. s 1 1o 10Normal-All Shot.

1 Delivered to bridge wire. This assumes no transformer impedance.

1 Firing voltage was 110 volts. 8 0.009 inch diameter.

Normal (firing)-No holes in the shells. Cap shell not broken at ignitionsection. H-Holes in the shells atagmtion section.

. CShells cut off at ignition section.

P-Plugs pushed or blown from the shells.

EXAMPLE HI Several firing tests were made, differing from thosedescribed with reference to tests 5 and 6 of Example II in that amatchhead type ignition composition, paraphenylenediaminedipicrate/potassium chlorate/Snow Floss 24/56/20, with 10%nitrocellulose (lacquer) was employed in lieu of the loose ignitionmixture of the caps tested. Snow Floss is a trade name for a highlypurified (substantially white) soft earthy formation of siliceigni-EXAMPLE V Several firing tests were made employing regular Result 10Normal-All Shot.

Do. Do. 40 Normal-All Shot.

Do. Do. Do Do. 20 Normal-All Shot.

Do. Do. Do. Do. Do Do Do No. of Trials an m w u n u B m mememeeememeeeoD DDDDDDDDDDDD 0 52 wwmw mwnw No. in Parallel the resulting gap beingabout inch.

EXAMPLE VI tion delay caps described with reference to Table 2 andmatchhead type ignition caps, the same as those of Table 2 except forthe matchhead igniter.

Table 5 FIRING VOLTAGE, 22o VOLTSI Result 4 NormalAll Shot. 5 Normal-AllShot.

Do. Do. 4 Normal-All Shot. 3 Normal-All Shot. 6 Holes-All Shot. 5Normal-All Shot. 2 Ncgmal-All Shot.

The following is a tabulation of data obtained when employing a copperlaminate utilizing a painted strip No.ol Trials No. of Trials Table 3No. in Parallel FIRING VOLTAGE, 220 VOLTS Line Resist.

Fuse Wire Nichrome N ichrome N ichrome EXAMPLE IV Test No.

ous skeletons of microscopic aquatic plants called diatoms. The data aresummarized as follows:

1 0.009 inch diameter-M length.

1 0.009 inch diameter-W length.

Several tests were made employing a metal clad laminate type fuse sealedwithin the cap, illustrated with m Q5 Q5 0 Rw0 0 0 0 5 0 fiw0 0 m 1 1111 m ii... n n a n u n q. o .1 h u m a m m. m. m I e e a R n M m n u o nn n N n n u a m n n e u T u u n 4 5 6 1 1 1 0.0056 inch diameterNichrome wire inch in length) connected each plate pair as speci icallyillustrated with reference to Figure 7a.

opposite ends, two separate copper plates,

Table 4 Line Resist.

Qua 0 0 5.0555 1 1 1 1 1 Length FIRING VOLTAGE, 220 VOLTS Fuse StripWidth Metal Ooppen.

Test No.

reference to Figure 5a of the drawings. The delay cap tested wasotherwise the same as that of the tests of Example 11.

9.-.. Copper;

13....... Steel...

1 Laminate W wide and /1 thick (paper base with phenolic resin binder)contained, copper plate 0.00135 inch thick, on each side, which wasreduced in width in a central section to form the fuse strip described.Each leg wire was secured to the copper plates by soldering, see Figure5a.

Normal (flring)-No holes in the shells. Cap shell not broken at ignitionsection. Holes-Holes in the shells at ignition section.

9 inch long) of conductive paint as the fuse element, as illustratedwith reference to Figure 8a, i.e., a fuse was disposed in each leadwire, within the cap. The firing conditions and cap assemblies wereotherwise the same as those described with reference to Table 2 (regular5 ignition) or Table 3 matchhead ignition.

that a metal clad laminate type fuse assembly was utilized in each legwire within the cap. Firing voltages were 220-440 volts. In tests 22 thedielectric plate, or strip, contained on the entire surface of eachside, a Nichrome foil 0.00016 inch thick. The foil as the fuse elementTable 6 FIRING VOLTAGE, 220 VOLTS Test No. Ignition Line No. in No. ofResult Resistance Parallel Trials l 20 N 1grrual--All Shot. Regular(loose ignition mixture) 17 (Pb/Se) (72.4/27.6)/Si/Snow Floss Q8 1 g3:97/2/11- 012 5 4 Do. Mthh dP h 0.2 20 1 Do.

a ea arap eny one 1- amine dipicratelpotassium chloi B2 18 rate/SnowFloss 24/56/20 with 10% 2 5 4 nitrocellulose (lacquer). Ba01/ 2 Se/SF(70/30/10) as auxiliary igui- 1 tion mixture.

EXAMPLE VII Several firing tests were made employing a delay electricblasting cap described with reference to Example II at firing voltagesof 220 and 440 volts, utilizing a copwas A: inch wide and inch inlength. The fuse assembly was the same in tests 23 except that the foilfuse element was Karma, and was inch wide and 0.0008 inch thick.

Table 8 Test No. Firing Line No.0f No. Ca s Result Voltage ResistanceTrials in Paral el 220 0.2 5 1 5N-All Shot. 22 15. 0 10 1 ION-All Shot440 0.2 5 1 5NAll Shot. 30. 0 10 1 10NAll Shot 220 0. 2 5 1 5NAll S11 2315. 0 10 1 7N, 3H-All Shot 440 0. 2 5 1 5N-All Shot.

30. 0 10 1 6N, 4H-All Shot.

N-Norrnal firing, no holes in the shells. HHoles in the shells at;ignition charges. per laminate fuse of Figure 7a 111 each leg wirewithin EXAMPLE IX the cap, i.e., a dielectric plate with a pair ofcopper plates secured on each side, separated from each other,

The following test data were obtained from firing a deand connected witha Nichrome fuse wire.

lay cap described with reference to Table 2 except that Table 7 Test No.Firin Line No. of No. Caps Result Voltag e Resistance Trials in Parallel0. 2 20 1 20 N-All Shot. l0. 0 20 1 Do. 20. 0 20 1 Do. 19 440 0 20 1 Do.0. 2 4 5 D0. 0. 2 2 10 D0. 0. 2 1 20 Do. 0. 2 10 1 10 N-All Shot its :21 s 0. so. 0 10 1 Do. 0. 2 2 5 Do. 0.2 1 10 Do. 0. 2 l0 1 Do. 13% is i13 o. 21 15. 0 1o 1 Do. 0. 2 2 5 D0. 0. 2 1 10 D0.

NNorrnal, no holes in the shells. Each Nichrome fuse wire was .0056 inchin diameter and inch long. The lamrnate (dielectric element) in eachinstance was inch long, 54; inch wide and he inch thlck. The caps oftest 20 were filled around the fuse composition with an asohaltwaterproofing composition aud topped with sulfur. The caps of tests 20and 21 were filled around the laminate use with Stypol (astyrene-polyester modified with vinyl toluene).

EXAMPLE VIII the fuse assembly was that described with reference toFigure 3a, i.e., the ignitlon plug was slotted so as to Data in Table 8were obtained from firing delay blastpartially cut through each wire pinto form the said fus :ing caps described with reference to Example 11except 7 assembly.

Table 9 Test No. Firing Line No. of No. Caps Result Voltage ResistanceTrials in Parallel 24 220 0. 2 1 ION-All Shot.

long. The original pln wires were #22 N lchrome wire, i.e., .025 inchdiameter.

EXAMPLE X The following tabulation shows the relationship of time ofcurrent flow to the elfect of the current on shell destruction. Asindicated, a period of from about 37-417 milliseconds (ms.) was utilizedduring which time all caps shot, although in some instances there wereholes formed in the shell of the ignition section. The data illustratethe necessity for the fuse or conductor section to fuse within a periodof about 120 ms. Thus, had the flow of initially applied current beenmaintained for a period in excess of about 120 ms., bursting of theshell and failure of the cap would have resulted.

In the foregoing Tables 1 and 3-10 the current, in each test, dependentupon the line resistance was as follows:

Current Amperes 1 Line Resistance, Ohms 220 Volts 440 Volts I Thisassumes no transformer impedance.

The diameter of the fuse element of the invention is any suitable valuewhich correlated with the bridge wire composition will fuse before thecap fails or bursts. Nichrome wire is the preferred fuse element, thediameter generally being within the range of about 0.0025 to about 0.142inch, preferably below about 0.009 inch.

Illustrative of preferred cross-sectional area values of fuse elementsof the invention are as follows:

Fuse Material Cross-sectional Area of Fuse Section, sq. in.

Steel .000093. Copper .0000135 Karma (Ni 73%, Cr, 20%+Al+Fe).-- .00005.Nlchrome (Cr/Nl/Fe, 60/16/24) .00001235-.0001583.

Although one fuse element can, in most instances, be satisfactorilyemployed in a blasting cap in accordance with invention, it is preferredthat a fuse element be included in each leg wire. If only one fuseelement is employed, there may occur an are between the leg wire andshell above the fuse with a resulting flow of current across theignition composition and on through the other leg wire to causedevelopment of an are followed by bursting of the cap, and failure.

When each wire contains a fuse element, there is a breaking of thecircuit even though there has been an are between the leg wire and theshell. Arcing between leg wire and shell can take place particularly inview of the close quarters in the cap shell to provide for a smalldistance in which the arc can be formed. However, use of one fuseelement is always satisfactory provided the leg wires are adequatelyinsulated from each other and from the shell wall.

As will be evident to those skilled in the art, various modificationscan be made or followed, in the light of the foregoing disclosure anddiscussion, without departing from the spirit or scope of the disclosureor from the scope of the claims.

What I claim and desire to protect by Letters Patent is:

1. In an electric blasting cap comprising a shell, a pair of leg wiresextending into said shell, a bridge wire in said shell connecting saidleg wires, and an ignition composition in said shell in operativecommunication with said bridge wire to be ignited by heat developed bypassage of electric current through said bridge wire, the improvementcomprising a fuse element inserted in at least one of said leg wireswithin said shell, said fuse element and said bridge wire each beingfusible within milliseconds at a temperature developed by heat formedfrom passage of electric firing current through said leg wires at avoltage sufficiently high to deliver at least 10 amperes to the saidbridge wire, and the diameter and fusion point of said bridge wire beingcorrelated with the diameter and fusion point of said fuse element sothat during said passage of firing current to deliver at least 10amperes to the said bridge wire, said bridge wire is caused to reach itsfusion temperature before said fuse element reaches its fusiontemperature, whereby during said 120 milliseconds period, said bridgewire is fusible prior to fusion of said fuse element.

2. A delay blasting cap of claim 1 containing said fuse element in eachof said leg wires.

3. In a delay blasting cap of claim 1, a portion of one of said legwires reduced in cross-sectional area as said fuse element.

4. In a delay blasting cap of claim 1, a wire as said fuse element.

5. In a delay blasting cap of claim 1, an assembly including said fuseelement comprising a dielectric material; said fuse element rigidlysupported on said dielectric material; and said leg wire, associatedwith said fuse element, also rigidly supported on said dielectricmaterial whereby said fuse element is supported free from tension fromsaid leg wire.

6. In a delay blasting cap of claim 5, first and second metal members,each electrically insulated from the other, affixed to said dielectricmaterial; a first portion of a leg wire affixed in electricallyconductive relation with one of said members, and a second portion ofsaid leg wire being aflixed in electrically conductive relation with theother of said metal members; and said fuse element aflixed inelectrically conductive relation with both of said metal members.

7. In a delay blasting cap of claim 6, a Nichromc wire as said fuseelement.

8. In a delay blasting cap of claim 6, a metal foil as said fuseelement.

9. In a delay blasting cap of claim 5, a metal member affixed to saiddielectric material, and separate portions of one of said leg wiresconnected with said member;

and a portion of said metal member intermediate th points connectingsaid leg wire portions, adapted as said fuse.

l0. In a delay blasting cap of claim 9, said intermediate portioncomprising a restricted portion of said metal member.

11. In a delay blasting cap of claim 5, a metal strip supported on saidelongated dielectric material, and said metal strip connected as saidfuse element with one of said leg wires as a section thereof.

12. In a delay blasting cap of claim 5, said assembly comprising a pairof metal strips, each as one said fuse, electrically insulated from eachother and afiixed to spaced apart areas on said dielectric material, anda first section of each leg wire aflixed to one end of each said stripand a remaining section of each said leg wire being afiixed to the otherend of each said strip.

13. In a delay blasting cap of claim 5, said assembly containing aconductive layer deposited on said dielectric material, as said fuse.

14. In a delay blasting cap of claim 13, said layer comprising a body ofmetal particles.

15. A delay blasting cap of claim containing said fuse element in eachof said leg wires.

16. The improvement of claim 15, wherein the said voltage is about 100volts.

17. The improvement of claim 15, wherein said voltage is about 220volts.

18. The improvement of claim 15, wherein said voltage is about 440volts.

19. An electric delay blasting cap adapted for firing at high voltagecurrents which comprises a shell; a pair of leg wires longitudinallyextending into said shell; an electrical resistance wire in said shellconnecting said leg wires; an ignition composition in said shell inoperative communication with said resistance wire to be ignited by heatdeveloped by passage of electric current through said resistance wire; abase detonatable explosive in said shell in an end of said shell awayfrom said lead wires; a primer composition intermediate said ignitioncomposition and said base charge and spaced from said base charge so asto detonate said base charge when heated; a slow burning heat-sensitivematerial as a delay fuse intermediate said ignition composition and saidprimer and ignitable by heat from ignition of said ignition composition;said fuse being in operative communication with said primer, whenburning, to cause detonation of same by heat from said burning; a fuseelement in at least one of said leg wires in said shell; said fuse andsaid resistance wire, each being fusible within milliseconds at atemperature developed by heat formed from passage of electric firingcurrent through said leg wires at a voltage sufficiently high to deliverat least 10 amperes to the said resistance wire, and the diameter andfusion point of said resistance wire being correlated with the diameterand fusion point of said fuse element so that during said passage offiring current to deliver at least 10 amperes to the said resistancewire, said resistance wire is caused to reach its fusion temperaturebefore said fuse element reaches its fusion temperature, whereby duringsaid 120 milliseconds period, said resistance wire is fusible prior tofusion of said fuse element.

References Cited in the file of this patent UNITED STATES PATENTS1,836,291 Schurmann Dec. 15, 1931 FOREIGN PATENTS 337,837 Great BritainNov. 10, 1930 611,915 Germany Apr. 9, 1935

